Heterojunction D- (or A+) millimeter and submillimeter wave detector

ABSTRACT

A solid state detector for use in detecting submillimeter and millimeter wave radiation. The solid state detector comprises a semiconductor superlattice of alternating thin epitaxial layers of GaAs and AlGaAs doped with impurities having the same conductivity type. Because of the transfer of charge carriers in order to obtain thermal equilibrium, the charge carriers in the higher energy doped layer transfer to the adjacent lower energy doped layer and form a predetermined number of either D -  or A +  centers in this layer. This D -  or A +  center formation is obtained without the use of an optical bias. The ionization energy for these D -  or A +  centers is small enough that photons from millimeter or submillimeter wave radiation is sufficient to ionize these carriers and yield a measurable conductivity in the device.

BACKGROUND OF THE INVENTION

The present invention is directed generally to solid state radiationdetectors, and more particularly to solid state detectors for thedetection of submillimeter, millimeter, or infrared wave radiation.

There is increasing interest in the use of millimeter and submillimeterwaves for military detection purposes. Wavelengths in the millimeter andsubmillimeter range have the advantage that they can penetrate smoke andclouds, while infrared waves cannot. Likewise, radiation in thiswavelength region provides good spatial resolution, while requiring onlya small antenna. These resolution and antenna features are in directcontrast to the requirements for microwave and radar wave devices.Additionally, all of the circuit components for millimeter andsubmillimeter wave devices are smaller than microwave and radar devicecomponents.

One of the major problems in fabricating a detector for millimeter andsubmillimeter waves is that the energy of the photon in thesewavelengths is proportional to frequency. As the frequency of theradiation decreases from the infrared region to the submillimeter regionto the millimeter region, the energy of the light photons decreases. Forexample, for the infrared wavelength of 10 microns, the energy perphoton is 120 meV. Note that this infrared wavelength is in the 8-12micron window. In contrast, for a wavelength of 1 millimeter (1000microns), the energy per photon is 1.2 meV. This is a reduction inenergy by two orders of magnitude from the energy of the infraredphoton. Thus a material must be found with bound charges which can beexcited by very small photon energies of on the order of 1.2 meV so thatmillimeter waves impinging on such a device would be sufficient toremove these bound charges to thereby increase the conductivity of thedevice. This change in conductivity could then be measured as anindication of the reception of the millimeter wave.

It is known that some doped semiconductors will form D⁻ and A⁺ centersunder certain circumstances. In this regard, neutral impurity dopingatoms can attract an additional charge carrier through the mechanism ofsharing the impurity atoms's charge with this extra charge carrier. A D⁻center is formed when a neutral impurity donor added to a semiconductorbinds not only the electron that it would normally bind, but also weaklybinds a second electron thereto. In the case of the donor atom, thissecond extra electron is bound via the sharing of the positive charge atthe atom's nucleus with the second electron. The energy binding thissecond electron to the neutral impurity donor is small enough that whena photon from a millimeter or submillimeter wave impinges on the D⁻center, this second weakly held electron is excited into the conductionband. Likewise, when a neutral acceptor impurity is added to asemiconductor, then the neutral acceptor atom will bind its own hole,and may also trap an extra hole very weakly. Accordingly, this A⁺ centerwith its trapped extra hole has a positive charge. Again, the very smallenergy obtained from the photon of a submillimeter or millimeter wavewill be sufficient to excite this second trapped hole into theconduction band for the material.

The basic problem for this type of device is that the number of steadystate D⁻ centers formed in a donor-doped semiconductor is very low.Likewise, the number of A⁺ centers formed in an acceptor-dopedsemiconductor is also very low. In order to increase the D⁻ centers orA⁺ centers so that when a millimeter or submillimeter wave impinges onthe material, a sufficient number of carriers will be excited into thematerial's conduction band so that a measurable response can bedetected, an optical bias is required. Typically, the dopedsemiconductor device is flooded with an infrared optical bias beam. Thisinfrared optical bias beam causes substantial photoconductivity, i.e.,many electrons are excited into the material's conduction band. Theseionized electrons then recombine with the various impurity centers inthe semiconductors material. Statistically, a certain percentage of thecarriers will combine weakly with neutral impurity atoms, resulting incharged impurity atoms. In the case of a donor impurity, a certainpercentage of the excited electrons will combine weakly with the neutraldonor impurity atom to form a D⁻ center.

The optical bias required to establish an appreciable steady state of D⁻or A⁺ centers results in a rather large, optically induced dark currentin the device. This large dark current caused by the optical bias, inturn, causes shot noise in the device which limits the performance ofthe device to infrared detection applications. This shot noise is thestatistical noise associated with the charge carriers moving from acrossone electrode to another in the device. The shot noise is proportionalto the square root of the dark current resulting from the optical biasand effectively acts to prevent accurate detection of small conductivitychanges in the device.

Accordingly, the problem confronting the art is how to minimize the darkcurrent in the semiconductor device to thereby minimize the shot noise.In essence, the problem is to form the D⁻ and A⁺ centers withoutflooding the semiconductor material with optical bias light which causesthe resulting dark current.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to substantiallyeliminate shot noise in millimeter and submillimeter wave detectors.

It is a further object of the present invention to provide a millimeteror submillimeter wave detector using D⁻ or A⁺ centers without utilizingoptical biasing.

Other objects, advantages, and novel features of the present inventionwill become apparent from the detailed description of the invention,which follows the summary.

SUMMARY OF THE INVENTION

Briefly, the present invention comprises a method and means fordetecting millimeter, submillimeter and infrared waves, comprising thesteps of disposing a specially designed semiconductor superlattice witha surface in a position to have millimeter, submillimeter, or infraredwave photons impinge on the superlattice surface, and then detecting thechange in conductivity in the superlattice. The specially designedsuperlattice comprises alternating thin epitaxial layers of at least twodifferent semiconductor materials, with the at least two differentmaterials having different intrinsic bandgaps, with the bandgapdifference therebetween being greater than KT, where K is the Boltzmannconstant and T is the detector operating temperature. These at least twosemiconductor materials are doped with neutral dopant molecules of thesame conductivity type, such that some carriers resulting from thedoping transfer from one of the doped thin epitaxial layers to anadjacent doped thin epitaxial layer and become weakly bound to neutraldopant molecules in the adjacent doped layer, i.e., D⁻ or A⁺ centers. Anincrease in the conductivity in the semiconductor superlattice resultswhen millimeter, submillimeter or infrared wave photons impinge on thesemiconductor superlattice and remove the weakly bound carriers from theD⁻ or A⁺ centers.

In a preferred embodiment, the semiconductor superlattice comprises atype I superlattice. This superlattice may be formed by alternatinglayers of GaAs and AlGaAs. These epitaxial layers should be in the rangeof 25-300 angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of the presentinvention.

FIG. 2 is a schematic diagram of the band structure of a dopedsemiconductor superlattice at the instant of formation.

FIG. 2b is a schematic diagram of the band structure of one embodimentof the superlattice of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is based on the use of a specially designedsemiconductor superlattice for radiation detection. It has been foundthat for superlattices comprising thin epitaxial layers of alternatingmaterials with different intrinsic bandgaps (e.g. GaAs, AlGaAs, GaAs . .. ) thermal equilibrium is achieved by transferring, charge carriers(either electrons or holes) from one layer to an adjacent lower energylayer. This feature of the superlattice is utilized in order to formcharged impurity centers. In essence, all of the alternating materiallayers are doped with the same conductivity type. In order to obtainthermal equilibrium, the charge carriers from a higher energy conductionband of one material layer transfer down to the lower energy conductionband of the adjacent layer. A significant percentage of these excesscharge carriers are then weakly bound to the neutral doped impuritymolecules to form the charged impurity centers. The ionization energyfor these carriers bound to the neutral impurity molecules is very smalland may be provided by photons from millimeter and submillimeter waves.Thus, the photoconductive response is obtained with this device in themillimeter and submillimeter radiation range without the requirement fora background optical bias.

Referring now to FIG. 1, there is shown a standard semiconductorsuperlattice. The device is comprised of alternating thin epitaxiallayers of at least two different semiconductor materials. Adjacentsemiconductor layers of different materials should have differentintrinsic bandgaps, with the intrinsic bandgap difference therebetweenbeing greater than KT, where K is the Boltzmann constant, and T is thedetector operating temperature. There are a variety of appropriatesemiconductor materials which may be utilized for these alternatingsemiconductor material layers. Typically, these materials will be chosenso that they are reasonably well lattice matched, i.e., the spacingbetween the atoms is approximately the same with the same geometricalatomic arrangement. By way of example, but not by way of limitation,FIG. 1 illustrates the superlattice as being comprised of alternatinglayers of GaAs and AlGaAs. The composition of AlGaAs is Al_(x) Ga_(1-x)As, where x may vary in the range 0≦x≦1, with 0.3 being one preferredvalue for x. Each of these layers should be very thin. Typically thelayer thickness will be in the range of 25-300 angstroms. For purposesof illustration only, the layers in FIG. 1 are shown as having athickness of 100 angstroms.

Referring now to FIG. 2a, the band structure for a superlattice ofalternating layers of AlGaAs and GaAs is shown. The bandgap for AlGaAsbetween its conduction band and its valence band is illustrated by thearrowed dimension 10. Likewise, the bandgap for GaAs between itsconduction band and its valence band is illustrated by the arroweddimension 12.

In accordance with the present invention, each of the layers of thesuperlattice is doped with an impurity of the same conductivity type.Typically, the doping is to a level of one part per 10⁶ -10⁹ molecules.If a D⁻ center device is desired, then donor impurities such as, by wayof example but not by way of limitation, Sn, Si, S, Se, Te, Ge, or otherdonors may be utilized. Likewise, if an A⁺ center device is desired,then acceptor impurities such as C, Be, Mg, Zn, Si (if disposed properlyin the material), Cd, Au, Mn, Ni, or other acceptors may be utilized. InFIG. 2a, doping with a donor impurity is illustrated in the figure. Thesmall dashes 14 are included in the drawing to illustrate the addeddonor impurities in each layer with energies near the conduction bandsfor the different layers.

As noted previously, in order for the superlattice to obtain thermalequilibrium, various donor carriers must transfer from one doped layerto an adjacent doped layer in order to find an equilibrium distributionwhich is characteristic for the operating temperature of the device.

For the band structure shown in FIG. 1a, the electron donor carrierspropagate or transfer from the higher energy conduction band of AlGaAslayer down to the the lower energy conduction band of the GaAs layer.These donor electrons now reside in the adjacent lower energy GaAslayer. A significant percentage of these excess electrons from theAlGaAs are then weakly bound to the neutral donor molecules in the GaAsto form D⁻ centers. The location of these extra electrons in theconduction band of the lower energy GaAs layers is illustrated in FIG.2b by the double set of dashed lines 16. Since these extra electrons areweakly bound to the donor molecules, the ionization energy of thesebound electrons is rather small and may be provided by the photons frommillimeter and submillimeter wave radiation. Accordingly, aphotoconductive response is produced for millimeter and submillimeterwave radiation.

There are a variety of techniques which may be utilized in order tofabricate a doped superlattice of the type used in the presentinvention. One preferred technique of fabrication is to use molecularbeam epitaxy. Using an MBE technique, a substrate such as GaAs, which isheated during growth, is located at the focal point of an 8-oven arraylocated on a circle. Each oven emits a beam flux of its particularheated and vaporized element. The above defined oven configurationallows all of the beams from the ovens to impinge on the GaAs target atthe same angle. High purity elemental charges such as As, Ga, Al, Be,and Si, are placed in the ovens for evaporation. The ovens may typicallybe Knudsen-type evaporation cells. The ovens operate at fixed, elevatedtemperatures (e.g., As at 350° C. Ga at 1070° C.), so that the fluxdensities of the beams impinging on the GaAs substrate can be controlledfor the proper growth desired. Layered thicknesses are controlled andhyperabrupt interfaces are formed by precisely opening and closing ashutter disposed in front of the opening for each of the ovens. Inoperation, in order to form a doped AlGaAs layer, the shutters in frontof the Al oven, the Ga oven, the As oven, and an impurity dopant ovenare opened to permit four simultaneous beams to impinge on GaAssubstrate target. By way of example, for an n-type dopant, the Si ovenmay be utilized. For a p-type dopant, the Be oven may be utilized. Whenthe proper thickness of the doped AlGaAs layer is obtained, than all ofthe oven shutters are closed. Then the shutters for the Ga oven and Asoven are opened along with an appropriate dopant oven shutter. Theselayers are then alternately grown on the substrate. This operation istypically performed in a supervacuum. The layer thicknesses aretypically on the order of 25-300 angstroms. It should be noted that GaAsand AlGaAs are particularly well suited to each other because they havevery close lattice matching which enables well-matched multilayerstructures to be grown.

It should be noted that in an preferred embodiment the superlatticeshould be a type I superlattice. Type I superlattices utilize two ormore different materials wherein the conduction band for each materialis always higher in energy than all of the valence bands for thematerials.

It should be noted that it is preferred that the present millimeter andsubmillimeter dectector device operate near absolute 0 temperature. Thepreferred temperature range is 0-4 degrees Kelvin, with a preferredtemperature of 2 degrees Kelvin or less.

In essence, the present invention provides a new solid state detectorfor submillimeter or millimeter-wave detection. This device utilizes D⁻or A⁺ center layers in a doped superlattice configuration. Such aconfiguration provides an appreciable concentration of D⁻ or A⁺ centersexisting in thermal equilibrium without any background optical bias.Accordingly, the shot noise for the device is greatly reduced in thisdesign. No other conduction processes are operating on the device whenradiation is absent. Thus this device is an insulator unless millimeteror submillimeter wave radiation is impinged thereon.

It should be noted that the spectral distribution of the photoconductiveresponse for the device can be varied by changing either the width ofthe material layers or the position of the D⁻ or A⁺ centers within thelayers. The positioning of the D⁻ or A⁺ centers in the layers can becontrolled simply by opening the donor or acceptor oven shutter at apredetermined time during the formation of the layer.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:
 1. A solid state detector for submillimeter and millimeter wave detection, which utilize no optical bias, comprising:a semiconductor superlattice comprising alternating thin epitaxial layers of at least two different semiconductor materials, with said at least two different alternating materials having different bandgaps with the intrinsic bandgap difference therebetween being greater than KT, where K is the Boltzmann constant and T is the detector operating temperature, wherein said at least two semiconductor materials are doped with dopant molecules of the same conductivity type, such that some carriers resulting from said doping transfer from one of said doped thin epitaxial layers to an adjacent doped thin epitaxial layer and become weakly bound to a dopant molecule in said adjacent doped layer.
 2. A detector as defined in claim 1, wherein said semiconductor superlattice is a Type I superlattice.
 3. A detector as defined in claim 2 wherein said at least two different semiconductor materials are GaAs and AlGaAs.
 4. A detector as defined in claim 3, wherein said dopant molecules are an n conductivity type.
 5. A detector as defined in claim 3, wherein said dopant molecules are a p conductivity type.
 6. A detector as defined in claim 2, wherein said thin epitaxial layers are in the range of 25-300 angstroms.
 7. A method for detecting millimeter, submillimeter, and infrared waves, comprising the steps of:disposing a semiconductor superlattice with a surface in a position to have millimeter, submillimeter, or infrared wave photons impinge on said superlattice surface, said superlattice comprising alternating thin epitaxial layers of at least two different semiconductor materials, with the at least two different materials having different intrinsic bandgaps, with the bandgap difference therebetween being greater than KT, where K is the Boltzmann constant and T is the detector operating temperature, wherein said at least two semiconductor materials are doped with neutral dopant molecules of the same conductivity type, such that some carriers resulting from said doping transfer from one of said doped thin epitaxial layers to an adjacent doped thin epitaxial layer and become weakly bound to a neutral dopant molecule in said adjacent doped layer; and detecting an increase in conductivity in said semiconductor superlattice when millimeter, submillimeter, or infrared wave photons impinge on said semiconductor superlattice and remove the weakly bound carries from said dopant molecules.
 8. A method as defined in claim 7, wherein said superlattice disposing step comprises the step of disposing a Type I superlattice.
 9. A method as defined in claim 8, wherein said superlattice disposing step comprises the step of disposing a superlattice with alternating layers of GaAs and AlGaAs with a layer thickness range of 25-300 angstroms.
 10. A method for detecting millimeter, submillimeter, and infrared waves with no optical bias, comprising the steps of:disposing a D⁻ semiconductor superlattice with a surface and with no optical biasing or pre-biasing in a position to have millimeter, submillimeter, and infrared wave photons impinge on said superlattice surface, said superlattice comprising alternating thin epitaxial layers of two different semiconductor materials, with said two different semiconductor materials having different bandgaps, with the bandgap difference therebetween being greater then KT, where K is the Boltzmann constant and T is the detector operating temperature, wherein at least alternating ones of said thin semiconductor layers are D⁻ center layers; and detecting an increase in conductivity in said semiconductor superlattice when millimeter, submillimeter, or infrared wave photons impinge on said D⁻ semiconductor superlattice.
 11. A method as defined in claim 10, wherein said superlattice disposing step comprises the step of disposing a Type I superlattice.
 12. A method as defined in claim 11, wherein said superlattice disposing step comprises the step of disposing a superlattice with alternating layers of GaAs and AlGaAs with a layer thickness range of 25-300 angstroms.
 13. A method for detecting millimeter, submillimeter, and infrared waves with no optical bias, comprising the steps of:disposing an A⁺ semiconductor superlattice with a surface and with no optical biasing or pre-biasing in a position to have millimeter, submillimeter, and infrared wave photons impinge on said superlattice surface, said superlattice comprising alternating thin epitaxial layers of two different semiconductor materials, with said two different semiconductor materials having different bandgaps, with the bandgap difference therebetween being greater than KT, where K is the Boltzmann constant and T is the detector operating temperature, wherein at least alternating ones of said thin semiconductor layers are A⁺ center layers; and detecting an increase in conductivity in said semiconductor superlattice when millimeter, submillimeter, or infrared wave photons impinge on said A⁺ semiconductor superlattice.
 14. A method as defined in claim 13, wherein said superlattice disposing step comprises the step of disposing a Type I superlattice.
 15. A method as defined in claim 14, wherein said superlattice disposing step comprises the step of disposing a superlattice with alternating layers of GaAs and AlGaAs with a layer thickness range of 25-300 angstroms. 